1. Field of the Invention
This invention relates to improvements in a piston for internal combustion engines, particularly an improvement in the circumferential surface of the piston.
2. Description of the Prior Art
A prior art piston is illustrated in FIG. 10. The circumferential surface of the piston 1 of an internal combustion engine is provided with piston-ring grooves 5, 6, 7 for mounting one or two pressure rings 2, 3 and ordinarily a single oil ring 4 in the order mentioned starting from the side of the combustion chamber 17. A first land 8 is formed between an end portion of the piston 1 on the side of the combustion chamber 17 and the first ring groove 5, a second land 10 is formed between the first ring groove 5 and the second ring groove 6, and a third land 13 is formed between the second ring groove 10 and the third ring groove 7. More specifically, in case of a two-ring type arrangement having a total of two rings in which there is one pressure ring 2, an oil-ring groove is situated on the lower side of the second land 10. In case of a three-type arrangement in which there are two pressure rings 5, 6, the oil-ring groove 7 is situated on the lower side of the third land 13, as shown in FIG. 10.
If a space (referred to as second-land space 18 or third-land space 19) bounded by the land immediately above the oil-ring groove 7, the inner surface of the cylinder 9 and the piston rings on either side is too small, there is an increase in pressure owing to oil scraped from the piston 1, and leakage of oil occurs on the side of the combustion chamber 17, as a result of which a large amount of oil is consumed. This tendency becomes particularly pronounced when two pressure rings are used.
Furthermore, when the pressure in the above-mentioned space referred to as the second-land space 18 or third-land space 19 suddenly declines, the oil is drawn up into this space via the abutting end faces of the oil ring, the oil pressure rises and the amount of oil consumed increases. When the pressure in this space suddenly rises, the pressure ring immediately overlying it is lifted up, thereby causing a decline in the sealing property of this pressure ring.
Accordingly, in order to prevent a sudden fluctuation in pressure within the land space, the practice in the prior art, which is illustrated in FIG. 10, is to provide the land surface of a land 13 with a flat step 15 that forms a recess toward the side of the central axis of the piston 1 on the side of an oil-ring groove 7. The land 13 is cut away uniformly below the step 20 to increase the land space 19. However, in such an arrangement in which the land is cut away uniformly on the upper side of the oil-ring groove 7, a problem arises in a case where the oil ring is a three-piece assembly comprising upper and lower rails and an expander. Specifically, the problem is that the seating of the upper rail is degraded, thereby leading to a decline in sealability.
In addition to the above-mentioned matter, the demand for high speed rotation, compactness and high load for the internal combustion engines has recently increased year by hear. Furthermore, local regulations for purification of exhaust gas from the internal combustion engines have become stricter. In order to decrease the amount of hydrocarbon in the exhaust gas, the top ring 2 is located near a top crown surface 21 of the piston 1 as much as possible so as to decrease a crevice volume of a top land space 22 bounded by the first land 8, the inner surface of the cylinder 9 and the top pressure ring 2.
It will be easily understood that the top pressure ring 2 must have a suitable thickness (T-dimension) and width (W-dimension) in view of a sealing property and strength of the top pressure ring 2.
In this high top ring piston, the high inertia force of the top pressure ring 2 results from the high speed rotation of the piston 1 and in breakage of the top land 8. The thin first or top land 8 is subjected to a high temperature from the combustion chamber so that a thermal strength thereof is decreased and the top land 8 is likely to be more easily broken. To prevent the breakage of the top land 8, the top ring 2 is decreased in width and an inertia force thereof is made smaller. However, this measure is not suitable for reduction of the amount of oil consumption.